The physical properties of homopolymers of propylene formed by typical Ziegler-Natta polymerization are highly dependent on the stereoregularity of the polymer itself. Highly stereoregular polymers are generally crystalline, provide desirable high flexural moduli and are formed with a suitable choice of electron donor. These highly crystalline polymers also display high melting points, but innately exhibit low melt flow rates (R) that render them generally unsuitable for applications that require high processing rates, such as in injection molding, oriented films and thermobond fibers. Further, conventional polypropylene homopolymer and copolymer products formed from highly crystalline polypropylenes lack sufficient impact resistance for many intended uses.
The addition of various electron donor materials to Ziegler-Natta catalysts has been known to influence the degree of stereoregularity in polypropylene homopolymers and copolymers. Generally, a single base catalyst (e.g., a magnesium chloride supported base Ziegler-Natta catalyst) can be used in combination with any number of electron donor materials, each of which, or combinations of which, will lead to a specific level of stereoregularity and MFR. One of the properties of electron donors is that the polypropylene MFR, at the same reactor hydrogen level, decreases with increasing polypropylene stereoregularity caused by the donor. Additional hydrogen is required to reach desirable MFRs when highly stereoregulating donors are employed.
Use of the term copolymer herein, is intended to mean those polymeric materials often known as impact copolymers. Impact copolymers typically include homopolymer polypropylene (homo PP) and an ethylene-propylene copolymer component. All references to the term copolymer are intended to include only impact copolymers (ICP) and not statistical or random copolymers. The production of such ICP's is discussed in Polypropylene Handbook, pg. 92, Hansen Publishers.
This relationship between stereoregularity and MFR poses a problem in processes that have hydrogen pressure limitations. In polypropylene manufacturing plants, hydrogen is used for as a chain transfer agent, which controls the molecular weight of the polypropylene. Molecular weight (MW) of the polypropylene decreases, i.e., the MFR of the polymer increases, with hydrogen concentration in a polymerization reactor. The phrase "MFR/hydrogen response" is typically used to express the efficiency of hydrogen as a chain transfer agent in the polymerization catalyst system process. A catalyst system with higher MFR/hydrogen response will generally produce higher MFR polypropylene at a fixed hydrogen concentration. The maximum MFR in the plant or process is defined by the MFR/hydrogen response of the catalyst system and the maximum H.sub.2 levels achievable. Higher MFR polypropylene has been required for larger scale injection molding processes producing items such as automotive or appliance parts. Such MFR's are typically in the range of from 30-100 dg/min. for copolymers, and 75-180 dg/min. for homopolymers,
On the other hand, polypropylene with higher crystallinity also has been required to enhance the stiffness and heat distortion temperature of the above discussed products. Crystallinity of polypropylene is defined by the catalyst system. To compare the ability of catalyst systems to produce higher crystallinity polypropylene, crystallinity of the polypropylene at fixed MFR can be used, because crystallinity of polypropylene increases with MFR of the polypropylene. Crystallinity is usually evaluated with heat of fusion (defined herein by (.DELTA.H.sub.m) or alternatively heat of melting) from DSC (Differential Scanning Calorimeter) measurements.
Polypropylene manufacturers have exerted substantial efforts over the past few years to develop a catalyst system which has both higher MFR/hydrogen response and the ability to produce higher crystallinity polypropylene. Higher MFR polypropylene with higher crystallinity is, for example, applied to automotive applications because larger scale injection molded parts with higher stiffness are required.
In processes wherein the desired hydrogen pressure cannot be reached, a more hydrogen responsive donor material must be used. However, the use of such donor materials leads to the formation of less stereoregular polypropylenes with more amorphous polypropylene by-products, producing polypropylenes that lack sufficient flexural moduli for the intended uses of an automotive trim part. Thus, processes that have hydrogen pressure limitations pose a problem because the final achievable MFR will be determined by the choice of electron donor, which in turn determines the level of polypropylene stereoregularity in the final product.
U.S. Pat. No. 5,652,303 ('303) to Ishimaru et al. suggests a method of forming a polypropylene in which propylene monomer is polymerized in the presence of a catalyst system including a solid titanium based Ziegler-Natta catalyst having an internal electron donor component, an organoaluminum compound cocatalyst and a blend of at least two electron donors including an electron donor (A) and an electron donor (B) wherein a relationship between the MFR (a) of homopropylene obtained by using electron donor (A), together with the specified titanium catalyst and organo-aluminum co-catalyst; and the MFR (b) of homopropylene obtained by using electron donor (B), together with the same catalyst and co-catalyst is defined by the equation: EQU log [MFR(b)/MFR(a)].gtoreq.1.5.
The use of a blend of two such electron donors purportedly leads to a broader molecular weight distribution (MWD), exemplified in '303 by MWD's in the range of 6.28 to 8.25, comparative examples ranging from 3.99 to 4.08. The broadening of the MWD allegedly leads to a polymer providing an improved processability.
Therefore, it would be highly desirable to synthesize polypropylene homopolymers in a single stage, in situ polymerization process that forms homopolymers that display both high flexural moduli and high MFRs. The present inventors have discovered that polymerization of an .alpha.-olefin in the presence of certain magnesium supported, titanium-base Ziegler-Natta catalyst systems and certain blends of electron donors will result in polymers simultaneously providing higher MFRs and higher levels of crystallinity, leading also to high modulus. It has been further found by the present inventors that the use of such blends of electron donors increases both MFR and crystallinity without substantially increasing MWD. The present invention also provides many additional advantages which shall become apparent as described below.